Have you seen this thread? Location: zigbeeOff event not triggering - #9 by velvetfoot
There should be some indication that ZB is offline in the location events section of the logs, and that should be an actionable event (sent alerts, reboot, etc.) I'm not sure you can get into MORE detail as to why from that specific log...
My initial suggestion, is to open a separate browser tab, and leave open the Settings->Zigbee Details->View log open to see if you can catch anything just before your crash (you'll likely want to disable your auto-reboots, as this log isn't persistant)
If more details is needed, either ask one of the HE support staff to look at the hubs engineering logs - And they may be able to sort out what device is the problem child (or a mesh interference issue) or if you more of a DIY sort of person, setup a Zsniffer (there are multiple threads on this), and monitor the mesh yourself, noting what's happening just before your Zigbee offline timestamp.